The invention relates to a power electronics unit, which comprises a controller and at least one half bridge with a first switching element and with a second switching element, and has a phase current output between the two switching elements, at which output the first switching element and the second switching element can be switched in the push-pull mode for a switching time at a cycle frequency, and at which the controller sets the switching time and/or the cycle frequency as a control variable, in order to provide, at the phase current output, a specified amplitude, frequency and phase position of the phase current within an imminent switching cycle.
Today's inverters for hybrid or electric vehicles usually use bipolar transistors with an insulated gate electrode (IGBTs) as semiconductors. This is indicated, for example, in the abstract of Chinese Patent Document CN 201 781 456 U. Inverters with bidirectional semi-conductor elements, such as metal oxide semiconductor field effect transistors (MOSFETs), as a rule, are used less frequently, because MOSFET switching bridges allow the inverting of only lower voltages than IGBT bridges. MOSFET inverters are therefore limited in their efficiency and IBGT inverters are preferred in this respect in many fields of electrical engineering; see, for example, German Patent Document DE 10138751 A1. However, at very high switching frequencies (>20 kHz), the component behavior of the MOSFETs has a positive effect in comparison to IGBTs.
A classical inverter consists of a B6 bridge known to a person skilled in the art. The two switches of one of the three half bridges are never switched on at the same point in time. A dead time therefore exists, which ensures that the switches can securely switch the input voltage and that there will be no half-bridge short circuit. In addition, during the conducting, voltage drops will occur at the semiconductors, which may have a disadvantageous effect on the operating performance of the inverter.
It is an object of the invention to describe an improved power electronics unit.
According to the invention, the amplitude, the frequency and the phase position of the phase current at the phase current output can be predicted; the direction of the predicted phase current is used as an observation variable of the controller and the controller determines the switching time for the imminent switching cycle as a function of the direction of the predicted phase current.
The controller is situated in a control circuit and determines the switching time to be set at the two switching elements. For technical reasons, there is a time delay between the point in time of the determination of the switching time and the point in time of the switching of the two switching elements. This means that the actual phase current at the point in time of the setting of the determined switching time of the switches does not correspond to the phase current that was the actual phase current at the point in time of the determination of the switching time. This leading of the phase current or the lagging of the determination of the switching time to be set for an imminent switching cycle can be compensated by the forecast calculation of the actual phase current present at the point in time of the setting of the switches and is called a prediction in the present document. The prediction therefore relates to a prediction time period, which essentially corresponds to the data processing time period in the control circuit of the controller and which describes the extent of the time of the lagging of the determination of the switching time to be set.
It is a preferred variant of the invention that a first switching time can be set for a positive direction of the predicted phase current, and a second switching time can be set for a negative direction of the predicted phase current.
The determined switching time is therefore a function of the direction of the predicted phase current. Voltage drops in the power electronics unit can therefore be taken into account which are a function of the current direction.
In addition, it is advantageous for a zero cycle to be determinable by means of the current prediction, in which zero cycle the phase current changes the current direction.
Without limiting the generality, a power electronics unit frequently emits an alternating-current signal. As an alternating quantity, the phase current necessarily assumes the zero value at a specified point in time, which zero value falls into a specified switching cycle. This switching cycle is called a zero cycle.
According to an embodiment of the invention, the determined switching time for the zero cycle is between the first switching time and the second switching time.
This means that the determined switching time for the zero cycle represents a hybrid between the first switching time for positive phase current and between the second switching time for negative phase current.
According to a preferred embodiment of the invention, the power electronics system is included in a system which also comprises an electric machine and in which the phase current of the power electronics unit is essentially used as an input current for the drive of a rotor of the electric machine and the prediction is essentially based on the rotation of the rotor during the prediction time period.
The invention is based on the considerations explained in the following:
Present inverters for hybrid and electric vehicles, as a rule, use IGBTs as semiconductor switches. MOSFET inverters are normally not used because MOSFETS only permit the switching of lower voltages. This limits the efficiency of the inverters. However, the electric strength of MOSFETS has increased considerably and hybridization concepts of vehicles are realistic in lower power ranges of approximately 10 kW, as, for example, for an expanded 48 V onboard power supply system for the additional start of an internal-combustion engine or for electric driving.
A classical inverter consists of a B6 bridge. In this case, it is disadvantageous that the two switches of one bridge should never be switched on at the same point in time in order to prevent a short-circuit of the d.c. input voltage. A dead time therefore exists, so that the switches can securely switch the direct voltage (usually present in the form of an intermediate circuit voltage UZk). In addition, voltage drops occur at the semiconductors and at the switching elements of the circuit connecting the semiconductors, which has a disadvantageous effect on the quality of the outgoing alternating current.
According to the state of the art, the voltage drops at the components of the inverter are only insufficiently taken into account in MOSFET inverters. As a result, the inverter impresses a different voltage into the machine, than the voltage that would correspond to the actual target specification. Because of the change in polarity of the phase current, the individual phase is connected approximately to either UZk or to the electric ground. This results in current harmonics in the machine, and no purely sinusoidal alternating-current phase is generated. In this case, the 5th and 7th harmonics in the current and therefore the 6th harmonic in the torque are significant. The Clark-Park Transformation, as the mathematical phase transformation, which will not be explained here in detail, is the basis of the interrelationship between current harmonics of the 5th frequency and the 7th frequency going into the electric machine and the mechanical wave of the 6th order at the machine output. The harmonics lead to undesirable torque fluctuations. Since they act similarly to an idle current component in the current, they result in additional power losses in the machine and in the inverter. In this case, the idle-time event in the case of MOSFET inverters differs significantly from those in the case of IGBT inverters since, in contrast to IGBTs, MOSFETs as switches conduct current in both directions and exhibit a different component behavior.
In the case of more complex controllers, which take into account the polarity of the phase current and possibly a voltage drop in the inverter connected with the respective polarity when determining the switching time, disadvantageous errors will occur when setting the switches in precisely those cycles in which the polarity of the phase current changes.
It is therefore suggested to improve the switching time of the inverter by a measures, which can be implemented by software, in the switching cycles with a change of polarity.
This provides a smoother torque and thereby a calmer behavior of the electric machine with respect to vibrations, fluctuations and their acoustics. In addition, the efficiency of the inverter will be improved.
In the following, a preferred implementation of the invention will be described by means of the attached drawings indicating further details, preferred embodiments and further developments of the invention. The same reference numbers describe identical technical characteristics.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.